This invention relates to laying a pipeline on the floor of a deep body of water, and more particularly to a method for arresting the propagation of any buckle in the pipeline that may occur while it is being laid or at any time thereafter.
During underwater pipelaying operations, a continuous length of pipe is payed out from a vessel into the water. The pipeline naturally bends and sometimes, either close to the vessel or in the sag bend, the buckling limit of the pipe can be exceeded, causing local buckling. Local buckling or plastic deformation of the pipe can also occur after the laying operation is completed due to falling objects (anchors or other heavy equipmeent) or natural causes such as earthquakes, sea bottom instabilities and undue currents.
When buckling occurs at any point in the pipeline, the collapse at that point may propagate along the pipeline for great distances in both directions due to hydrostatic pressure. It is desirable to prevent the propagation of a buckle over too large a section in order to minimize the length of pipe that must then be replaced. One method of arresting a buckle is disclosed in U.S. Pat. No. 3,747,356. The technique described there is to place a cylinder inside the pipe tethered to a cable payed out with the pipeline to allow the cylinder to remain in a section of pipe until that section reaches the floor. The cable length is then fixed so that an additional pipe is payed out, the cylinder is pulled along through the inside of the pipe. Any buckle that may occur in the pipeline between the vessel and the floor is thus immediately arrested and not allowed to propagate along the pipeline already resting on the floor. The problem with this technique is that it will not prevent a buckle that may occur in the pipeline, after it is laid, from propagating along the floor.
One could place hollow cylinders inside the pipeline as buckle arrestors at selected intervals, and leave the cylinders in place, but that would reduce the effective internal diameter of the pipeline. It would be possible to place permanent cylinders or sleeves outside the pipe at regular intervals as described by U.S. Pat. No. 3,768,269, to simply increase the section modulus of the pipeline for restricted lengths at intermittent intervals. It should be noted that the first (internal) method has been shown to be more efficient than the second because the hydrostatic pressure acts only on the pipe and not on the arrestor, whereas in the second (external) method the hydrostatic pressure acts on the arrestor itself.
Both of these arresting devices have the disadvantage that they cannot be used in the case of a continuous pipelaying process, such as one where the pipe is laid from a shipboard reel onto which the pipeline has been prewound. This is because no end exists through which the sleeves could be inserted or over which the sleeves could be slipped. Another disadvantage with these arrestors is that, due to their construction, they increase the bending rigidity of the pipe so that, at the ends of these sleeves or heavier sections, excessive local stresses are apt to occur due to the discontinuity in thickness, which can create problems.